WASHINGTON (AFP) - The United States does not want to be the "policeman" of the Middle East, US President Donald Trump tweeted on Thursday (Dec 20), as he defended his controversial decision to pull US forces out of Syria.

"Does the USA want to be the Policeman of the Middle East, getting NOTHING but spending precious lives and trillions of dollars protecting others who, in almost all cases, do not appreciate what we are doing? Do we want to be there forever? Time for others to finally fight...," he tweeted.

Mr Trump added: "Russia, Iran, Syria & many others are not happy about the US leaving, despite what the Fake News says, because now they will have to fight ISIS and others, who they hate, without us."

"I am building by far the most powerful military in the world. ISIS hits us, they are doomed!"

The US President faced intense opposition to his abrupt announcement on Wednesday that Islamic State in Iraq and Syria (ISIS) had been defeated in the region, and that he was ordering the 2,000 US troops in Syria to exit the country.

Mr Trump's decision runs counter to long-established US policy for Syria and the region. It blindsided lawmakers, the Pentagon and international allies alike.

"Getting out of Syria was no surprise," Mr Trump wrote in an earlier tweet. "I've been campaigning on it for years, and six months ago, when I very publicly wanted to do it, I agreed to stay longer."

"Russia, Iran, Syria & others are the local enemy of ISIS. We were doing their work. Time to come home & rebuild. #MAGA"

The US withdrawal could have major geopolitical ramifications, and plunges into uncertainty the fate of US-backed Kurdish fighters who have been tackling ISIS extremists, thousands of whom are thought to remain in Syria.